The present invention relates to optical semiconductor devices that have optical semiconductor elements and relates, in particular, to an optical semiconductor device which can be utilized for an optical communication device that transmits and receives optical signals by means of an optical transmission medium of an optical fiber or the like.
The optical semiconductor device of the present invention is used for electronic equipment such as digital TV (Television) sets, digital BS (Broadcasting Satellite) tuners, CS (Communication Satellite) tuners, DVD (Digital Versatile Disc) players, CD (Compact Disc) players, AV (Audio Visual) amplifiers, audio devices, personal computers (hereinafter referred to as PC's), personal computer peripherals, portable telephones and PDA (Personal Digital Assistant) devices. Moreover, the optical semiconductor device can also be used for electronic equipment such as automobile onboard equipment of car audio devices, car navigation devices and sensors, sensors of robots in factories and control equipment, the equipment used in environments of wide operating temperature ranges.
An optical semiconductor device that connects optical semiconductor elements of a light-emitting diode (LED) and a photodiode (PD) with an optical fiber cable has conventionally been known and used for optical communications between devices and in homes and automobiles.
As these optical semiconductor devices, as shown in FIG. 31, those which are fabricated by utilizing transfer molding with a transparent resin are widely used. In the optical semiconductor device shown in FIG. 31, an optical semiconductor element 102 bonded onto a lead frame 101 with a conductive adhesive paste 103 is encapsulated with a transparent resin 106, and the optical semiconductor element 102 and the optical fiber cable 111 are optically connected with each other by a lens 110 formed of the translucent resin. The optical semiconductor element 102 is electrically connected to the lead frame 101 by a wire 104. Moreover, an integrated circuit chip 105 for controlling the driving of the optical semiconductor element 102 is mounted bonded onto the lead frame 101 with the conductive adhesive paste 103.
In general, the transparent resin used for an optical semiconductor device as described above has had a problem that the coefficient of linear expansion thereof has been increased by the use of the transparent resin filled with no filler making great account of the optical characteristics, which has led to a problem in the environmental resistance (thermal shock resistance, heat dissipation and so on).
Therefore, an optical semiconductor device (shown in FIG. 32), which can be encapsulated with a mold resin filled with filler by devising the construction of the optical semiconductor device, is disclosed (refer to, for example, JP 2000-173947 A). In the optical semiconductor device shown in FIG. 32, a glass lens 212 is stuck to only the optical part of an optical semiconductor element 202, the element is mounted on a lead frame 201 with a conductive adhesive paste 203, and an electrode located at the periphery of the optical part of the optical semiconductor element 202 is electrically connected to the lead frame 201 via a wire 204. Subsequently, by transfer forming with a mold resin filled with filler, the optical semiconductor element 202 and the wire 203 can be encapsulated with a mold resin portion 207 without shielding an optical path through which light enters and goes out of the optical semiconductor element 202.
Moreover, as a semiconductor device resin encapsulating technique, as shown in FIG. 33, a resin encapsulating technique for providing a first encapsulating resin portion 308 that integrally encapsulates the main body constituents including a lead frame 301, a semiconductor element 302 bonded onto the lead frame 301 with a conductive adhesive paste 303 and a bonding wire 304 that connects these members, and a second encapsulating resin portion 309 that is formed to cover at least part of the outer peripheral portion of the first encapsulating resin portion 308 and selecting the first and second encapsulating resin portions 308 and 309 so that the coefficient of linear expansion of the second encapsulating resin portion 309 is made smaller than the coefficient of linear expansion of the first encapsulating resin portion 308 is also disclosed (refer to, for example, JP 04-92459 A).
Since the conventional optical semiconductor device is fabricated by transfer molding with the transparent resin filled with no filler, there are great differences in the coefficient of linear expansion among the transparent resin, the lead frame, the optical semiconductor element and the bonding wire, and this leads to a problem that the troubles of wire disconnection, package cracking and so on occur due to thermal stresses. There is a further problem that the transparent resin has a thermal conductivity of about 0.17 W/m·K, which is much smaller than that of a metal (e.g., a copper material has a thermal conductivity of 365 W/m·K), and this makes it difficult to dissipate heat generated in the optical semiconductor element, limiting the operating range at high temperature. Due to the problems, it is very difficult to fabricate a highly reliable optical semiconductor device.
Moreover, it is known that the coefficient of linear expansion and the thermal conductivity can be adjusted by filling the mold resin with filler. However, since it is difficult to achieve filling of filler (or it is permissible to achieve filling of filler only by a small amount) due to a reduction in the light transmittance in the optical semiconductor device of which the optical characteristics are valued, there has been a problem in fabricating a highly reliable optical semiconductor device. Therefore, in order to use a mold resin filled with filler, as shown in FIG. 32, a structure in which a glass lens is mounted in the light-receiving portion of an optical semiconductor element and the lens is partially peripherally encapsulated with a resin, can be considered. However, actually in this structure, a glass lens can be placed in the optical part when an optical semiconductor element of a comparatively large size (several millimeters to several tens of millimeters square) as in a CCD. In contrast to this, in an optical semiconductor element of a small size (several hundreds of micrometers square) as in an LED, a very small glass lens needs to be used since the optical part is very small, and there are the problems that:
(i) it is difficult to fabricate a minute glass lens;
(ii) it is difficult to achieve mutual bonding and alignment between the optical part and the glass lens; and
(iii) interfacial separation occurs due to a difference in the coefficient of linear expansion between the glass and the mold resin due to thermal stresses.
Moreover, there is a problem that the wire bonding cannot be carried out when a glass lens that is larger than the optical part of the optical semiconductor element is employed since the glass lens overlaps the electrode located adjacent to the optical part of the optical semiconductor element. Moreover, another method for a structure in which a semiconductor device electrically connected to a lead frame with a bonding wire is covered a first encapsulating resin, and the peripheral portion of the first encapsulating resin is further covered with a second encapsulating resin, as disclosed in FIG. 33, by which the coefficient of linear expansion of the second encapsulating resin is made smaller than the coefficient of linear expansion of the first encapsulating resin to reduce the separation between the encapsulating resins due to thermal stresses, is also considered. However, since the coefficient of linear expansion of the first encapsulating resin is greater than the coefficient of linear expansion of the second encapsulating resin, there occurs a problem that the first encapsulating resin is formed in a swelled state due to heat at the time of forming the second encapsulating resin, and the contraction of the first encapsulating resin becomes greater than that of the second encapsulating resin at the time of cooling after the formation, causing separation at the resin interface and reducing the reliability of the moisture resistance and so on.